Radio-frequency identification (RFID) is the wireless use of radio-frequency electromagnetic fields to transfer data, often used to automatically identify and/or track objects via RFID devices attached to or otherwise associated with the objects. The RFID devices may contain electronically stored information. Some devices are powered by and read at short ranges (a few centimeters to a few meters) via magnetic fields (electromagnetic induction). These devices typically have no battery. Rather, they collect energy from the interrogating electromagnetic field. Other devices may use one or more local power sources such as a battery and then act as a transponder to emit radio waves. Battery powered tags may operate at up to hundreds of meters. RFID tags may be included in seals as described in U.S. Pat. No. 8,282,013, incorporated by reference herein in its entirety.
Injection molding often utilizes a ram or screw-type plunger to force melted polymer into a cavity of a mold (molds can include a single cavity or multiple cavities). The polymer solidifies into a shape conforming to the mold cavity. In multiple cavity molds, each cavity can be identical (to form the same parts) or can be unique (to form different geometries during a single cycle). Molds may be made of any suitable material, but are generally made from tool steels. Stainless steels and aluminum molds are suitable for certain applications. Although it wears faster, aluminum may be cost effective in low volume applications because mold fabrication costs and time can be considerably lower.
Pelletized raw thermoplastics, one type of polymer, may be fed through a hopper into a heated chamber with a reciprocating screw. The temperature increases and the Van der Waals forces that resist relative flow of individual chains in the material are weakened as a result of increased space between molecules. This reduces the material's viscosity, enabling the polymer to flow and be driven by the injection unit.
The reciprocating screw may deliver the raw material forward, mixing the polymer and reducing the required heating time by mechanically shearing the polymer and adding a significant amount of frictional heat. The material may be fed through a check valve and may collect at the front of the reciprocating screw into a volume known as a shot. The amount of material in a shot typically is sufficient to fill the mold cavity, compensate for shrinkage, and provide a cushion (approximately 10% of the total shot volume may remain in the chamber to prevent the screw from bottoming out) to transfer pressure from the screw to the mold cavity.
When enough material has gathered, the material may be forced at high pressure and velocity into the mold cavity. Injection times are usually quite quick, often well under 1 second. Packing pressure is applied to complete mold filling and compensate for thermal shrinkage. The packing pressure may be applied until the cavity entrance solidifies.
Next, the screw reciprocates and acquires material for the next cycle while the material within the mold cools, solidifying enough to be ejected. Cooling lines circulating fluid may reduce the required cooling time. Once cooled, the mold opens and an array of pins, sleeves, strippers, etc. may be driven forward to extract the molded element. Then, the mold closes and the process may be repeated.